1. Field of the Invention
This invention relates to a rotor for electric motors, and more particularly to such a rotor provided with a frame molded from a resin for holding a rotor yoke on which rotor magnets are disposed.
2. Description of the Prior Art
Japanese Patent Publication No. 2-211046A (1990) discloses a rotor of the above-described type. FIG. 22 shows the disclosed rotor. The rotor comprises a frame 1 made of a synthetic resin by way of the injection molding. The frame 1 comprises a cylindrical yoke mounting portion 2 and a base or thin sheet portion 3 having a generally V-shaped section. The thin sheet portion 3 includes a integrally formed cylindrical shaft supporting portion 4. A rotational shaft 5 of the rotor is inserted through the shaft supporting portion 4 to be supported.
An annular rotor yoke 6 and a plurality of rotor magnets 7 are mounted on the yoke mounting portion 2. The rotor yoke 6 and the rotor magnets 7 are accommodated in a molding die assembly and a molten resin is then poured into the die assembly so that the rotor yoke 6 and the rotor magnets 7 are integrated with the frame 1. The rotor yoke 6 is made by roll ingands-tacking band steel sheets.
In the above-described construction, the rotor yoke 6 is subjected heat during the injection molding of the frame 1 so that the rotor yoke is deformed in the direction of arrow A in FIG. 22 or in such a direction that the diameter thereof is increased. On the other hand, the diameter of the rotor yoke 6 is reduced in the direction of arrow B opposite arrow A during the cooling of the frame 1 subsequent to the molding. As the results of these deformations and further a difference between heat shrinkage rates of the rotor yoke 6 and the frame 1, the thin sheet portion 3 is subjected to stress during the cooling subsequent to the molding, whereupon the shaft supporting portion 4 is axially deformed. Although the molding die assembly is designed so as to cope with the deformation of the shaft supporting portion 4, the deformation is unstable and accordingly, a sufficient dimensional accuracy cannot be achieved. With this, since the stress produced during the shrinkage and expansion of the thin sheet portion 3 remains alive, cracks etc. tend to occur.